MVS/370 also used absolute zero for the IPL processor. In MVS/XA, we changed to use a non-zero prefix for every processor.
Jim Mulder z/OS Diagnosis, Design, Development, Test IBM Corp. Poughkeepsie NY "IBM Mainframe Assembler List" <ASSEMBLER-LIST@LISTSERV.UGA.EDU> wrote on 11/12/2019 01:51:22 PM: > From: "Seymour J Metz" <sme...@gmu.edu> > To: ASSEMBLER-LIST@LISTSERV.UGA.EDU > Date: 11/13/2019 01:14 AM > Subject: Re: Questionable Instructions in Obtaining EAX documentation > Sent by: "IBM Mainframe Assembler List" <ASSEMBLER-LIST@LISTSERV.UGA.EDU> > > I don't know about MVS, but OS/360 support for 65MP used absolute > address 0 for one processor. > > > -- > Shmuel (Seymour J.) Metz > http://mason.gmu.edu/~smetz3 > > > ________________________________________ > From: IBM Mainframe Assembler List <ASSEMBLER-LIST@LISTSERV.UGA.EDU> > on behalf of John McKown <john.archie.mck...@gmail.com> > Sent: Tuesday, November 12, 2019 9:49 AM > To: ASSEMBLER-LIST@LISTSERV.UGA.EDU > Subject: Re: Questionable Instructions in Obtaining EAX documentation > > On Tue, Nov 12, 2019 at 8:43 AM Tom Marchant < > 000000a69b48f3bb-dmarc-requ...@listserv.uga.edu> wrote: > > > >I cannot fathom the reason to use _any_ base for the PSA other than GPR0. > > >It is simply wasteful of a scarce resource. > > > > It isn't "the" PSA. There is the PSA of the processor that you are running > > on. > > That PSA is always at location 0. > > > > True. I left out the words "of the processor". I guess that I ASSuMEd that. > > > > > > > Every processor in the LPAR has its own PSA, and when the processor is > > running, > > that PSA is at real (and virtual) location 0 for that processor. The PSA > > for any > > other processor can also be referenced using the real address that matches > > the > > value that is used for the prefix register for that processor. This is > > documented > > in the Principles of Operation under "Prefixing". > > > > I know. But I have never wanted to look at the PSA of any CP other than the > one that I am running on. I'm not really sure why I would. Do you know of a > reason to do so? I am curious. > > > > > > > You cannot use the value in the prefix register for your own processor to > > reference > > your PSA though. That would get you to absolute page 0, which, if I > > understand > > correctly, MVS never uses. > > > > That's interesting. So z/OS and it's ancestors have never really used > absolute page 0. I guess it doesn't really matter since it is a small > amount of physical storage in today's world. Actually, I thought absolute 0 > was used as the PSA for the IPL processor, but that, again, was an > assumption. > > > > > > > -- > > Tom Marchant
